# what happens when both anode and cathode voltages are same for a diode?

In a circuit if the ideal diode has both the cathode and anode voltage same, will it conduct or not? Similarly, in a constant drop model if the voltage drop is exactly equal to 0.7 will it conduct or not?

• For the first case, think about it for a second: whatever component you have, if it has no voltage potential there can be nothing to conduct. Commented Aug 19, 2015 at 8:14

This is actually a good question, and should not have been downvoted. The answer is that it could be any (forward) current (including zero)- you cannot tell what the current is from the voltage. Same as with the ideal wires we draw on a schematic. That is a consequence of the 'ideal' assumption- it's not a physical reality.

If you are trying to analyze a circuit with ideal diodes you have to treat them sort of like wires that are there if the voltage drop across them would be in the correct direction if they were removed.

The only time this comes up in reality is with a superconducting wire- if there is zero volts (exactly zero volts, not too low to measure) then there could be +/- any current through the wire (up to some limit in reality).

If the voltages at two points are exactly the same, you can put any passive element between them and no current will flow through that element. So you could put a wire, a resistor or a diode there, nothing will change.

If one point is exactly 0.7V above the other and you put a diode in between in the forward direction with a stated forward voltage of 0.7V some current will flow. A Diode is specified at a certain current, be that 0.1mA, 1mA, 10mA or 50A, so if you put that known voltage across it, and the diode is exactly as specified, a current very near that specified current will flow.

If you make the voltage a little lower a lower current will flow, if you make it higher, a higher current will flow.

Look at this little plot:

(source: richardson at csserver.evansville.edu)

You can see that at 0.7V this diode (random plot I found from a guy that did a test on a diode) conducts about 15mA. Which seems to be about its first regular operating point, so it serves your question well enough. Then if you make the voltage 0.6V, you can see still about 1mA will flow. But if you make it 0.8V it will go off the plot.

When thinking about discontinuities such as you encounter in an ideal diode, it's useful to think about approaching the discontinuity from the left or from the right. If you are approaching 0V from the left (0-), resistance is infinite, and there is no current. If you are approaching zero volts from the right, resistance is zero, but there is still no current, as there is no potential difference between anode and cathode.

If both side of diode are connected with same terminal and having same voltage value then there will be no conduction hence no current however if potential difference is equal to or greater than conduction threshold I.e 0.7 for germanium then there will be conduction I.e if anode is connected with 1.7+ve and cathode is 1 v +ve then diode will conduct but both +ve voltages should be from different batteries same phenomenon is used in comparator which is used to make a cutoff circuit for battery charging

• Welcome to EE stack exchange. Note that this question is actually 2 years old and already has an accepted answer. It may be better to focus on questions which don't have a good answer yet. Commented Oct 21, 2017 at 6:13

When Ideal diode is connected to the same voltage on both terminals then there will be no current .. but it is not true in all cases .. yes the diode (in a circuit) can be on if also there is same voltage across both terminals.! Refer the circuit in the figure given below where the diode is ideal and the current through diode is 1A case where it has same voltage on both terminals. This question was designed by IIT Madras professors in GATE exam 1997 (https://i.sstatic.net/NxqiE.jpg)